Magnetic circuit



July 14, 1959 J (:LUWEN 2,895,092

MAGNETIC CIRCUIT Filed April 16, 1957 Fl GJI awn FIG.4

INVENTOR JOHANNES MEYER CLUWEN United States Patent Ofitice 2,895,092Patented July 14, 1959 MAGNETIC CIRCUIT Johannes Meijer Cluwen,Eindhoven, Netherlands, as-

signor to North American Philips Company, Inc., New York, N.Y., 'acorporation of Delaware Application April 1 6, 1957, Serial No. 653,119Claims priority, application Germany April 18, 1956 8 Claims. (Cl.317--201) This invention relates to magnetic circuits comprising apermanent main magnet, one or more permanent auxiliary magnets made fromdifferent material, and a softmagnetic part for the transmission of themagnetic flux produced by the permanent magnets to an operating oractive point. Such circuits are used, for example, in loudspeakers andin magnetic electron lenses respectively, the field set up at theoperating point acting upon a loudspeaker moving coil and an electronbeam to be focussed, respectively. However, the invention can generallyalso be used in bias magnetisation arrangements.

In such magnetic circuits, use is generally made of permanent magnetshaving a high magnetic output, which are included in the circuit so thatthey are operated at their maximum BH-product or peak energy product(where B is the magnetic induction and H is the magnetic fieldstrengthwithin the magnet). Thus, the magnet produces a magnetic potential and amagnetic flux which is transmitted by the soft magnetic part to theoperating point. The magnetic potential drop along the soft magneticpart gives rise to stray or leakage fluxes between various points ofthis part, so that only a materially smaller flux arrives at theoperating point.

It is an object of the invention to increase the effective flux and/ orto reduce the stray flux. To this end, it can be ensured by a suitabledesign of the magnetic circuit that about one-half of the fiux producedby the permanent main magnet arrives at the operating or active point.

It is also an object of the invention to increase further the magneticflux produced at the operating point and/ or to reduce further the strayflux produced outside the operating area. This object can be realized bycompensating this stray field by a field of the same strengthoriginating from at least one auxiliary magnet made from a differentpermanent magnet material, having a diiierent operative fieldstrengthand being arranged at at least one point at which a comparatively strongstray flux is produced.

A magnetic circuit in accordance with the invention is characterized inthat the main magnet and the auxiliary magnets are magnetizedperpendicularly to each other, and in that the materials from which themagnets are made are different, in the sense that the fieldstrength ofthe main magnet and the auxiliary magnet respectively at that point ofthe BH-curve or magnetization characteristic-where the product of theinduction B and the fieldstrength H becomes maximum-(BI D is essentiallydifferent, and that the fieldstrength of the auxiliary magnetcorresponding with its (BH) -product is substantially the. same as thestray fieldstrength produced by the mainmagnet at theplace of theauxiliary magnet,

increased, as the case may be, by its own demagnetization nent magnetmaterial enables the auxiliary magnet to be operated so that it providesits maximum magnetic output. (Generally, in carrying 'out this step thelength of the magnet measured in the direction of magnetization must bemade slightly greater in order to adjust the magnetic circuit so thatits BH-product is a maximum.) Preferably the auxiliary magnet isarranged so that the stray flux produced by the main magnet is alsoreduced. However, this compensation, which can only be partial, does notprovide any further increase of the effective flux. In order that theinvention may readily be carried out, some embodiments thereof will nowbe described, by way of example, with reference to the accompanyingdiagrammatic drawings, in which:

Fig. 1 shows a magnetic circuit for use in a loudspeaker,

Fig. 2 is a modification of the circuit shown in Fig. 1,

Fig. 3 shows a magnetic circuit for a loudspeaker of different design,

Fig. 4 shows a third embodiment of such a magnetic circuit and Fig. 5shows a magnetic circuit for an electron lens.

The loudspeaker magnetic circuit shown in Fig. 1 comprises a cylindricalaxially magnetised permanent main magnet 1 made of a material having ahigh magnetic output but a comparatively low operative fieldstrength.Use may, for example, be made of the material known under the trade nameTiconal which has an operative fieldstrength H of 500 oersted and anoperative induction B of 10,000 gauss. The magnet 1 is provided with asoft magnetic pole hood 2 and a cylindrical soft magnetic shell 3between which an airgap 4 is produced in which a loudspeaker moving coilcan be arranged.

Between the magnet 1 and the shell 3 a comparatively strong stray fieldis produced, which, particularly in the proximity of the airgap 4, maymaterially exceed the operative field strength produced within themagnet 1. Owing to the high value of the stray fieldstrength, withrespect to the fieldstrength within the main magnet, the provision of anauxiliary permanent magnet made of a similar material would give littleadvantage, since the operating point of this auxiliary permanent magnetwould be highly unfavourable, for, as is well-known, a permanent magnethas its greatest eifect when it is operated at that point of theBH-curve where its energy product is at its peak. In order to reducethis stray field and to increase the effective field produced in theairgap 4, provision is made of an annular radially magnetized auxiliarymagnet 5 made of a magnetically stronger material, for example,Ferroxdure, having a fieldstrength of about 1250 oersted and aninduction of about 1700 gauss at the operating point Where the productBH of the induction and the fieldstrength becomes a maximum. Accordingto the invention the auxiliary magnet 5 is operated at substantially itsmaximum magnetic output, that is to say at its peak energy product, in amanner such that the fieldstrength of the auxiliary magnet 5corresponding with its (BI-D -product is substantially the same as thestray fieldstrength produced by the main magnet 1 at the place of theauxiliary magnet 5. The field of this auxiliary magnet 5 is added to thefield of the main magnet 1 and provides a real increase of, say, from20% to 30% of the effective field in the airgap 4, and hence of theeffective flux. The gain in flux may be eliminated while maintaining thesame magnetic potential byan increase in the magnetic reluctance of themain magnet 1. The main magnet 1 is accordingly prolonged and can bemade substantially more slender as is shown in Fig. 2. Consequently, alarger space becomes available for the auxiliary magnet 5 in Fig. 2 sothat it may be made from Ferroxdure having a field strength of 1000oersted and an induction of 2200 gauss at its operating point and henceprovides an even greater contribution to the effective flux.

Since the stray fieldstrength decreases in the direction towards the endremote from. the airgap 4, the magnet 5 may, if required, be tapered andthe shell 3 may be conical (not shown) Fig. 3 shows a loudspeakermagnetic circuit comprising a disc-shaped axially magnetised Ferroxduremain magnet 1 and a soft magnetic part constituted by discs a and 10 andby a central cylindrical core 11 which together with the disc 9 boundsthe airgap 4. Between the outer circumferences of the discs 9 and 10 aconsiderable stray flux is produced, however, its fieldstrength is lessthan the operative fieldstrength of the main magnet 1.

By the provision of two annular radially magnetised auxiliary magnets 12and 13, made for example from Ticonal', the effective field produced inthe air-gap 4 is increased. At the same time the circumferential strayfield of the main magnet 1 is partially compensated for by the magnetsl2- and 13, so that there is less interference inelectronic apparatusarranged in the proximity of the circuit. Since the auxiliary magnets 12and 13 lie in an open magnetic circuit, use must be made of a permanentmagnet material having an operative fieldstrength which is substantiallyequal to the total sum of the stray fieldstrength and the naturaldemagnetisation fieldstrength produced by the cylindricalcircumferences. If required, this latter step can be dispersed with orcan be assisted by the provision of an annular auxiliary magnetencircling the main magnet 1 and being magnetized in the same directionbut having a lower operative field strength.

In the embodiment shown in Pig. 4, the main magnet 1 of higher operativefieldstrength, which may be made from Ferroxdure, is tubular andmagnetized radially. The soft-magnetic parts comprise a rod-shaped core11 and a cylindrical shell 3 the upper ends of which are spaced apart toform an air-gap 4. The stray flux at their lower ends can be materiallyreduced by the provision of an annular and/or a rod-shaped axiallymagnetised magnet 16 and/ or 17 respectively of lower operativefieldstrength. (Owing to the lesser concentration of lines of magneticforce, the magnet 16 can usually be dispensed with.)

A similar effect is achieved by the auxiliary magnet 20 of the magneticelectron lens shown in Fig. 5. This lens comprises two disc-shaped mainmagnets 21 and 22 axially magnetised in opposite directions andsoft-magnetic discshaped parts 23, 24 and 25, between which anelectrontocussing field is set up along the optical axis 26. The discs23 and 25 have equal magnetic potentials and consequently do not giverise to a mutual stray flux. At the circumference of the discs only, aconsiderable stray flux is produced from the discs 23 and 25 to the disc24, however, this stray flux is at least partially compensated for atthe point of highest concentration of the lines of magnetic force bymeans of the auxiliary magnet 20 having a lower operative fieldstrength.

The material Ticonal as mentioned herebefore, is described in US. PatentNo. 2,295,082.

Ferroxdure materials are oxidic permanent magnetic materials, theconstituents essential to the permanent magnetic properties of which arecompounds having a hexagonal crystal structure and a chemical formula MCa 0.6Fe O in which 0.6 x l and in which M stands for one or more of themetals Ba (barium), Sr (strontium) and Pb (lead), these materials beingmore amply described in US. Patent No. 2,762,777.

What is claimed is:

1. A magnetic circuit comprising main and auxiliary permanent magnetsarranged so that their magnetization directions are substantially atright angles to one another and a soft magnetic member for carrying theflux of the magnets to an active point, said main and auxiliary magnetsbeing composed of materials having magnetization characterics such thattheir field strengths at which their energy products are a maximum aredifferent, said auxiliary magnet being located at a position in thecircuit where the sum of its own demagnetizing field strength and thestray field strength produced at that position by the main magnetsubstantially matches the field strength of the auxiliary magnet atwhich its energy product is a maximum.

2. A circuit as set forth in claim 1 wherein one of the magnets has ahigh field strength and low induction at its peak energy product, andthe other magnet has a low field strength and a high induction at itspeak energy product.

3'. A magnetic circuit comprising main and auxiliary permanent magnetsarranged so that their magnetization directions are substantially atright angles to one another and a soft magnetic member for carrying theflux of the magnets to an active point, said main and auxiliary magnetsbeing composed of materials having magnetization characteristics suchthat their field strengths at which their energy products are a maximumare different, said auxiliary magnet being located at a position in thecircuit where the sum of its own demagnetizing field strength and thestray field strength produced at that position by the main magnetsubstantially matches the field strength of the auxiliary magnet atwhich its energy product is a maximum, said auxiliary magnet having ahigher field strength at its operating point than that of the mainmagnet.

4'. A circuit as set forth in claim 3 wherein the main magnet isrod-shaped and is magnetized axially, and the auxiliary magnet isannular and surrounds the main magnet and is magnetized radially.

5'. A magnetic circuit comprising main and auxiliary permanent magnetsarranged so that their magnetization directions are substantially atright angles to one another and a soft magnetic member for carrying theflux of the magnets to an active point, said main and auxiliary magnetsbeing composed of materials having magnetization characteristics suchthat their field strengths at which their energy products are a maximumare different, said auxiliary magnet being located at a position in thecircuit where the sum of its own demagnetizing field strength and thestray field strength produced at that position by the main magnetsubstantially matches the field strength of the auxiliary magnet atwhich its energy product is a maximum, said auxiliary magnet having alower field strength at its Operating point than that of the mainmagnet.

6-. A circuit as set forth in claim 5 wherein the main magnet isdisc-shaped and is magnetized axially, and the auxiliary magnet isannular and magnetized radially.

7. A circuit as set forth in claim 6 wherein soft iron discs areprovided on opposite sides of the main magnet, and the auxiliary magnetsurrounds one of these soft iron discs.

8. A circuit as set forth in claim 5, wherein the main magnet is tubularand magnetized radially, and the auxiliary magnet is rod-shaped andmagnetized axially.

References Cited in the file of this patent UNITED STATES PATENTS2,398,653 Linlor Apr. 16, 1946

